Decibel Calculations

Amplifier gain figures are often given in decibels, and we may encounter feedback referred to in decibels as well. It is, however, always more convenient to make the calculation in ratios and convert the result to decibels. This avoids confusion.

In the negative feedback example that we discussed, the gain of the amplifier without feedback was 10,000 or 80 db. The feedback fraction was 9/10,000, or approximately -61 db. Hence, the loop gain A(J is 80 - 61 = 19 db. This corresponds to the loop gain ratio of 9. The feedback factor is 1 + 9 or 10, which is 20 db. So in this case, the loop gain is 19 db and the feedback is 20 db. For negative feedback, the bigger the loop gain, the nearer the two figures come to coinciding. If the loop gain were 99 (39.9 db), the feedback would be 100 (40 db).